This invention relates to an apparatus for testing a subscriber's line. More particularly, the invention relates to a subscriber's line testing apparatus for testing subscriber's lines accommodated by a DLC (digital loop carrier).
In sparsely populated areas where residences are dispersed at comparatively great distances, a switch (exchange) cannot be provided at each residence because of the costs involved. Accordingly, a switch is provided in one town, a DLC is provided on the switch side and on the side of each residence, and the DLC on the residence side and the DLC on the switch side are connected by a time-shared multiplex transmission line.
A plurality of subscriber's terminals such as an ordinary household telephone, a public telephone and a data terminal are connected to the DLC on the residence side via analog subscriber's lines (referred to as "terminal-side subscriber's lines"). The DLC on the side of the residence subjects an analog signal, which enters from each terminal-side subscriber's line, to an A/D conversion, and sends the digital signal, which has been obtained by the A/D conversion, to a digital transmission line upon subjecting the digital signal to time-shared multiplexing. Further, the DLC on the side of the residence demultiplexes the time-shared multiplexed signal that has entered from the digital transmission line, subjects the separated digital signal to a D/A conversion and sends the analog signal to the terminal-side subscriber's line.
The DLC on the switch side is connected to the switch via a plurality of analog subscriber's lines (referred to as "switch-side subscriber's lines"). This DLC subjects an analog signal, which enters from the switch via each switch-side subscriber's line, to an A/D conversion, and sends the digital signal, which has been obtained by the A/D conversion, to a digital transmission line upon subjecting the digital signal to time-shared multiplexing. Further, the DLC on the switch side demultiplexes the time-shared multiplexed signal that has entered from the digital transmission line, subjects the separated digital signal to a D/A conversion and transmits the resulting analog signal to the switch-side subscriber's line.
The switch-side subscriber's lines and the terminal-side subscriber's lines are in 1:1 correspondence. Regardless of the fact that the DLC on the switch side and the DLC on the terminal side are interconnected by a single digital transmission line, functionally it is just as if the subscriber's terminals and switch were interconnected independently by respective subscriber's lines. As a result, in a case where there is a great distance between the place of installation of the switch and each residence, an advantage obtained is that the cost of laying cables can be reduced by using the DLCs.
FIG. 18 is a block diagram illustrating the configuration of a communication system that employs DLCs. The system includes a switch 10, a switch-side DLC 20, a terminal-side DLC 30 provided at each residence, subscriber's terminals 40, a time-shared multiplexed digital transmission line 50, switch-side subscriber's lines 60a.about.60n, and terminal-side subscriber's lines 70a.about.70n.
The switch-side DLC [DLC(COT), where "COT" is the abbreviation of "central office terminal"] and the terminal-side DLC [DLC(RT), where "RT" is the abbreviation of "remote terminal"] are identical in construction. Channel panels (CH panels) 21a.about.21n, 31a.about.31n are provided for respective ones of the subscriber's lines. Also provided are multiplexer/demultiplexers (MUX/DMUX) 22, 32 and interfaces 23, 33. The CH panels convert analog signals, which enter from the subscriber's lines 60a.about.60n and 70a.about.70n, into digital signals and output the digital signals to the digital-transmission side. Further, the CH panels convert digital signals, which have entered from the side of the digital transmission line, into analog signals and send the analog signals to the subscriber's lines 60a.about.60n and 70a.about.70n.
The multiplexer/demultiplexers (MUX/DMUX) 22, 32 each have a multiplexer and a demultiplexer. The multiplexer outputs n-items of digital data, which are the result of digital conversion by each CH panel, to the digital-transmission side upon subjecting the data to time-shared multiplexing. The demultiplexer demultiplexes the time-shared multiplexed data, which has entered from the side of the digital transmission line, and inputs the demultiplexed data to each CH panel. Interfaces 23, 33 convert the unipolar time-shared shared multiplexed signal, which has entered from the corresponding multiplexer, into a bipolar time-shared multiplexed signal and inputs the signal to the digital transmission line 50. The interfaces 23, 33 also perform a bipolar-unipolar conversion, which is the reverse of the foregoing conversion, and input the resulting signals to the corresponding demultiplexers.
Ordinarily, n-number of subscriber's lines (where n=24) are connected to each DLC, 24 lines of digital data are subjected to time-shared multiplexing and the resulting data is sent to the opposing DLC in the form of a bipolar signal at a transmission rate of 1.5M. FIG. 19 is a diagram of frame composition, in which one frame is composed of 24 time slots and digital data on 1st.about.24th lines (channels) is inserted into 1st.about.24th time slots (Time Slots #1.about.190 24). A frame bit F is placed at the head of each frame, and one super frame is composed of 12 frames. An A bit and a B bit serve as signaling bits of each channel. The A bit is inserted in an eighth bit of each time slot in the 6th frame (DSI Frame #6), and the B bit is inserted in the eighth bit of each time slot in the 12th frame (DSI Frame #12). It is possible to communicate the status of a telephone by combining the signaling bits A and B and sending them, when the combination is denoted by (A, B), the off-hook status is communicated by rendering the (A, B) same as (0,0), on-hook status (1,0), IDLE status (1,1) and ringing status (1, alternating 1/0). Here "alternating 1/0" means that 1,0 alternate.
FIG. 20 is a block diagram illustrating the construction of the CH panel. TIP and RING represent subscriber's lines, which are connected to an outside-line metallic cable. More specifically, in a case where the DLC is provided on the terminal side, the subscriber's line is connected to a subscriber's terminal (mainly a telephone). In a case where the DLC is provided on the switch side, the subscriber's line is connected to the switch. BAT represents a subscriber's feeder that supplies a current via the TIP/RING lines in order to drive the subscriber's terminal (private equipment such as a telephone). ON/OFF HOOK DET represents an on-hook/off-hook status detector which, by monitoring the value of the current that flows through the TIP/RING lines, detects the on-hook/off-hook state of the subscriber's terminal. RINGING GEN represents a ringing signal generator for generating a ringing control signal to ring a bell on the subscriber's terminal. Further, 2W/4W represents a two-wire/four-wire converter for converting the TIP/RING lines, in which a transmitted signal and a received signal are fully duplexed, into four individual lines on the transmitting and receiving sides. The converter 2W/4W is constituted by a converting transformer and a balancing network.
TX AMP denotes a transmitting-side controller for converting the level of an analog signal on the transmitting side into a stipulated value of the analog input level of a CODEC, described below. RX AMP represents a receiving-side amplifier for converting the level of the output analog signal of the CODEC into a stipulated value of the analog input level of the subscriber's terminal. The CODEC is a coder/decoder for effecting a conversion between analog and digital signals, inserting a signaling bit at conversion of a digital signal and extracting the signaling bit at the conversion to an analog signal. SIGNALING DET generates a signaling bit that is in accordance with a CPU command and notifies the CPU of the signaling bit extracted by the CODEC. TX/RX represent a pair of digital signal lines on transmitting and receiving sides. The digital signal sent from the lines TX are multiplexed in 24 channels on the multiplexer/demultiplexer which is interfaced with the side of the digital transmission line 50. The multiplexer/demultiplexer demultiplexes the multiplexed signal and inputs the demultiplexed signal to each line RX. The CPU is for control of the CH panel and ordinarily executes various signaling bit processing such as commanding the generation of a signaling bit corresponding to the status of the subscriber's terminal, commanding the transmission of a control signal to the subscriber's terminal conforming to a received signaling bit, etc.
FIGS. 21 and 21A show a diagram for describing operation when the on-hook state is in effect (i.e., when the subscriber is not in a telephone conversation). Numeral 80 denotes a repeater.
1 The switch 10 applies a normal battery value (a current value that prevails when a conversation is not being carried out) to the CH panel (COT) 21 from the feeder.
2 The CH panel (COT) 21 detects that the normal battery value is being supplied from the switch 10 and sends an IDLE code (in which the A and B bits are both "1") to the CH panel (RT) 31.
3 The CH panel (RT) 31 detects the IDLE code, applies the normal battery value to the subscriber's terminal (telephone) 40 and transmits an on-hook code (in which the A and B bits are both "0") to the CH panel (COT) 21.
When the on-hook state is in effect, steps 123 above are executed continuously.
FIGS. 22 and 22A show a diagram for describing the placing of an outgoing call.
1 When the handset of the telephone 40 is lifted in the on-hook state, the load within the telephone changes and so does the battery value.
2 The CH panel (RT) 31 monitors the change in the battery value. The CH panel 31 detects a change in the battery value to recognize that the off-hook state has been attained and then transmits an off-hook code (in which the A bit is "1" and the B bit is "0") to the CH panel (COT) 21.
3 The CH panel (COT) 21 detects the off-hook code and shows the switch 10 a load which is the same as that of the telephone 40. As a result, the battery value changes and the switch 10 recognizes that the off-hook state has been attained.
4 Next, dial pulses are sent from the telephone 40 to the switch 10.
5 After transmission of the dial pulses, the CH panel (COT) 21 sends the IDLE code (1,1) to the CH panel (RT) 31 so that a transition is made to a state in which conversation is possible.
FIGS. 23 and 23A show a diagram for describing operation when an incoming call is terminated.
1 When an outgoing call is placed to the telephone 40 from another subscriber's terminal in the on-hook state, the switch 10 accommodating the telephone 40 sends a ringing signal to the CH panel (COT) 21.
2 After detecting the ringing signal, the CH panel (COT) 21 transmits a RINGING code (in which the A bit is 1 and the B bit is "alternating 1/0") to the CH panel (RT) 31.
3 When the CH panel (RT) 31 detects the ringing code, it sends a ringing signal to the telephone 40 so that the latter issues a ringing tone.
4 When the terminating party lifts the handset to take it off the hook in response to the ringing tone, the battery value changes.
5 The CH panel (RT) 31 detects that the battery value has changed, recognizes that the off-hook state is in effect and transmits the off-hook code (1,0) to the CH panel (COT) 21.
6 Upon detecting the off-hook code, the CH panel (COT) 21 changes the battery value and informs the switch 10 of the fact that the telephone 40 has been taken off the hook.
7 In response to notification of the fact that the telephone 40 has been taken off the hook, the switch halts the transmission of the ringing signal.
8 The CH panel (COT) 21 recognizes that the ringing signal has been halted and transmits the IDLE code to the CH panel (RT) 31, whereby a transition is made to the state in which a telephone conversion can be carried out.
FIGS. 24 and 24A show a diagram for describing operation at the end of a conversation.
1 When a conversation ends and the handset is hung up (placed on the hook), the battery value changes to the normal battery value.
2 Upon detecting the normal battery value, the CH panel (RT) 31 transmits the on-hook code (1,0) to the CH panel (COT) 21.
3 Upon detecting the on-hook code, the CH panel (COT) 21 makes the load shown to the switch the same as the load that prevails when the telephone 40 is in the on-hook state and returns the battery value to the normal battery value. As a result, the switch 10 recognizes that the conversation has ended.
It is described above that the number of subscriber's lines connected to one DLC is 24. However, this number approaches 200 in a system of the largest scale. When a failure occurs in a line at a DLC that is already being provided with service from the switch 10, maintenance itself is very difficult to perform since there are so many lines connected to one DLC. Moreover, an operation for recognizing the section of the faulty location involves a great amount of labor. Consequently, in order to prevent an initial failure and perform a level diagnosis adjustment, it is necessary that the DLC be tested successively, section by section, from the installation stage.
There are two methods of performing a conventional test of subscriber's lines accommodated by a DLC. (1) The first testing method involves measuring a test signal, which is outputted directly by a switch, using a measuring device connected in place of the subscriber's telephone, whereby the status of the line is ascertained. (2) The second testing method involves attaching a jack to each CH panel mounted on the DLC, connecting a measuring device to each opposing CH panel via the jack, outputting a test signal to the digital transmission line from the measuring device of the CH panel on one side in a state in which the connection to the switch is cut off, and measuring the test signal using the measuring device of the opposing CH panel, whereby the status of the line is ascertained.
In the above-described conventional testing methods, testing cannot be carried out unless a cable (digital transmission line) is laid between the DLCs.
Further, even if testing is performed upon laying a cable, normality/abnormality through the entirety of a subscriber's line is merely judged, and the section of the faulty location where a line has failed cannot be recognized.
Furthermore, in the first testing method of the prior art described above, it is necessary to change the connection of the testing device each time the line tested is changed. In the second testing method, the connection of the measuring device must be changed at both of the opposing CH panels. This makes the test very troublesome.